Frequency modulation signaling system



Sept 17, 1940- E. H. ARMSTRONG 2,215,284

FREQUENCY MODULATION SIGNALING SYSTEM INVENTOR l [dw/f7 hf Army/*ongSept 17, 1940- E. H, ARMSTRONG 2,215,234

FREQUENCY MODULATION SIGNALING SYSTEM Sept. 17, 1940.

E. H. ARMSTRONG FREQUENCY MODULATION SIGNALING "SYSTEM Filed Feb. 19,1940 6 Sheets-Sheet 3 inve/ope 0f Moda/017270 [We/ope 0f DefecfofCarre/lf AT TORNE YS Sept. 17, 1940. E. H. ARMSTRONG FREQUENCYMODULATION SIGNALING SYSTEM Filed Feb. 19, 1940 6 Sheets-Sheet 6Res/afer frequency/f7 C.

INVENTOR,

Patented Sept. 17, 1940 PATENT OFFICE FREQUENCY MoDULA'rIoN sIGNAmNG l ySYSTEM Edwin H. Armstrong, New York, N. Y. Aplplication February 19,1940, Serial No.1319,569

nrc yJ, 7 1940 s claims. (or. 25o-6) This invention relates toimprovements in frequency modulation transmission systems in radiosignaling. It has for its object the improvement of the signal to noiseratio and the `improvement of the quality of transmission. It isparticularly useful for very high fidelity transmission of music. Theinvention relates to the introduction of a distorting network at thetransmitter and a restoring network at the receiver together with the 10provision of certain detector characteristics in such manner as toproduce a lgreat improvement in the quality of transmission.

Referring now to the figures which form a part of this specification,Fig. 1 illustrates the general 1:, arrangement of the transmitter andFig. 2 the general arrangement of the receiver. Fig. 3 illustrates thearrangement of a predistorting network used at the transmitter and Fig.4 a restoring or converse network. Fig. 5 shows the electricalcharacteristics of these networks. Figs. 6,

7, 8 and 9 are a series of characteristics for thev purpose ofexplaining certain operations occurring within the system. Fig. 10illustrates the details of a modulating arrangement and Fig. 11

shows the characteristics of this arrangement. Figs. 12, 13, 14 and 15illustrate some of the characteristic operations of the receiving systemand Figs. 16 and 17 show a simplified form of the predistortion andrestoring networks.

'Ihe difficulty which the invention proposes to overcome may beunderstood from the following explanation. It is well known that toproduce realism in the reproduction of music that the total audiblefrequency range must be trans- 35 mitted and reproduced. This range isfrom perhaps 30 or 40 cycles per second to 15,000 or 16,000

cycles per second. It is also well known that present radio practicefalls far short of this, barely half of this range being transmittedand,

40 for various reasons, less than half of this vrange being reproduced.It is also well known that in addition to failure to transmit the totalfrequency range that the present amplitude modulation methods introduceharmonics as a result of dis- 45 tortion in certain places in thetransmitting and receiving systems. It is for these reasons that thepresent-day radio sets sound like a radio.

It is also well known that when an attempt is madelto transmit thehigher frequencies of the 50 musical range in the ordinary manner, thewidening of the band width of the receiver to accommodate the higherfrequencies of the transmitted wave results in the admission of muchadditional noise. It is well known that the dis- 55 tribution of theenergy among the various frequency components of the voice or of anorchestra is such that the greater part of it is concentrated in thelower frequency range so that REISSUED the higher frequencies arerelatively much weaker Y than the lower ones. As a consequence of thiswhen the transmitted wave is substantially fully modulatedby the lowerfrequencies,- the depth of 'modulation for the higher frequencies isrelatively very small.

are the flrst to be lost in the noise. It is well known that thisdifficulty can be remedied to a considerable extent by the introductionof a distorting network at the transmitter which is4 so designed that itraises the higher frequencies to substantially the same level as thelower ones so that they may more effectively override the noise and bythe introduction of a converse or restoring network at the receiverwhich recreates the proper relation between the various frequencycomponents of -the transmission. It has been found that when thisprocess is applied to a wide range frequency 'modulation system itproduces a much greater gain than in an amplitude modulation system. It,however, greatly accentuates the distortion or creation of harmonicswhichhas heretofore been referred to and produces a very unpleasant typeof reproduction. It is the purpose of this invention to show how thisdifficulty may be avoided and the process applied to frequencymodulation transmission with an increase in both the fidelity and noiselevel greater than can be obtained with the process as applied to theordinary method of transmission.

Referring now to Fig. 1, which illustrates the `general arrangement ofthe transmitter, I represents the microphone, 2 the usual preamplifier,3 the predistorter, an amplifier of the speech frequencies, 5 themodulator of a frequency modulation transmission system, 6 a series offrequency multipliers exciting a finalpower amplier stage 1 which inturn' feeds the radiating system 8. The frequency modulation transmittercomponents 5, 8 and 1 maylbe of the type described in my U. S. Patent#1,941,068. l

Referring now to Fig. 2, I0 represents the receiving antenna, Il anamplifier for thereceived current, and I2, I3 and I4 the usualconverter, oscillator and intermediate frequency amplifier of asuperheterodyne receiver. I5 represents a limiter for removing amplitudemodulation, I6 a filter, and I1 an amplifier for the filter output. Thenetworks I8, 20, 22 and I9, 2|, 23 are for the purpose of convertingfrequency changes into amplitude changes and the circuit is similar tothat described in my U. S. Patent #1,941,069. 24,

Hence the higher 'frequencies 23 and 23, 21 represent detectors arrangedacross each half of the circuit, and 3l represents a bal-- anced outputtransformer connecting into the restorer 3l the audio ampliiier 32 andthe speaker or other translating device 33.

The circuits of `the predistorter and restorer are illustratedrespectively in Fig. 3 and 4. 'I'he transmission characteristics ofthese two networks are illustrated by curves A and B, respectively, inFig. 5. As shown by curve A, the frequencies around 15,000 cycles areraised by the predistorter to some ten times the amplitude of the lowestfrequencies, and in the restorer as shown by curve B these highfrequencies are cut down an exactly similar amount so as to give anoverall uniform transmission characteristic for transmitter andreceiver. Fig. 6 shows the envelope of modulation as it appears in anamplitude modulation system at the transmitter under normalcircumstances, and Fig. 7 shows the same characteristic as modiiied whenpredistortion is included in the transmission. Fig. 8 illustrates theenvelope of the current in the detecting system, and Fig. 9 shows thecurrent voltage curve of the ordinary detector. Fig. 10 shows thedetailed circuit arrangement of the modulator 5 of Fig. 1, and Fig. 11certain oi' its characteristics. Fig. 12 shows the characteristics ofthe networks I8, 20, 22 and I9, 2|, 23 of Fig. 2 under one condition ofadjustment, and Fig. 13 shows the characteristics of two networks havinga somewhat different design. Fig. 14 illustrates the comparative noisevoltage distribution with respect to frequency in the detector systemsof an amplitude and frequency modulation receiver, respectively, andFig. 15 shows the relative improvements which are obtained when therestorer is applied to the two systems.

It is now in order to consider problem for which this inventionfurnishes a solution. Referring now vto Fig. 6, there is illustrated theenvelope of the modulation of an amplitude modulated transmitter. Thecurve shows the variation of the low frequency modulating voltage, therather complex wave form indicating a deep low frequency modulation witha shallow modulation of higher frequency tones. It will be understood,of course, that this curve represents likewise the peak value of theradio frequency currents radiated. It is .essential in amthe particularplitude modulation in order to produce the best signal to noise ratio tooperate the transmitter as nearly as possible at full modulation. Toaccomplish this, the gain of the modulation ampliiier is set so that thedepth of the modulation approaches the zero line as closely aspossible'without actually touching it. Points A AI in Fig. 6 illustratethis condition.

In actual practice, of course, it is very diiii'- cult to preventovermodulation even with the most careful supervision, and although thelarger or low frequency modulations may be kept from overshooting thezero line the higher frequency ripples will at times inevitablyovexshoot it. This means distortion and the production of harmonics,particularly of the frequencies composing the ripples on the mainmodulation,.and the value of these may reach relatively high levelswithrespect to the levels of the fundamental ripple frequencies. As aconsequence of this. a disagreable harsh tinge will be introduced in thesound reproduction. Now when predistortion is inserted in the modulatingsystem so that the high frequency ripples are increased in amplituderelatively to the low frequency ones, this form of and distortion isgreatly aggravated. The' general effect is as illustrated in Fig. '1,where points B and B1 show the accentuation of the over modulation, andconsequently of the distortion. The curves have been drawn in a way toillustrate cxtreme cases of distortion. It is, however, not necessary toactually overshoot the zero line to produce it. Distortion is likewiseencountered when the depth of the modulation is such that nearlycomplete modulation is produced because the voltage-current curves ofvacuum tube ampliiiers o r oscillators are not strictly linear but bendat their lower end, so that the envelope oi the radio frequency currentis n ot strictly proportional to the wave form of the modulatingvoltage. This type of distortion is almost always present, unlessrelatively low percentages of modulation are used.

At 'the receiving end various .forms of distortion occur. v A certainamount is due to lack of linear amplication but for the moment this maybe disregarded in View of a more disturbing form. Fig. 8 shows theenvelope of the radio frequency current in the rectier or detectorcircuit. It will be observed from observation of the points C and C1that two forms of distortion may occur. One is that due to the lack oflinearity of the detector characteristic as shown in Fig. 9, and the.other is actual destruction of the form of the fine ripples by crossmodulation with the noise components., The last form may be very badindeed. In the application of the predistortion method to amplitudemodulation it becomes necessary, therefore, to reduce the percentagemodulation at the transmitter to prevent distortion. This results in arise in the noise level so that the net theoretical gain of the systemis substantially reduced.

Examining now the sources of distortion at the transmitting end ofafrequency modulation system, particularly the phase shifting frequencymultiplying type described in my U. S. Patent #1,941,068, it will befound' that distortion due to lack of linearity in tube characteristicscan be eliminated. Figs. 10 and l1 show how the distortion due to lackof linearity in the characteristics of the tubes used for modulating maybe avoided. Fig. 10 shows a general form of phase shifter which may beused. represents the source of exciting current which is applied to thegrid of the carrier amplifier 36 and to the tuned circuit 38 through aphase adjusting network 31, so arranged that the E. M. F.s applied tothe grids of 39 and 40 are 90 out of phase from the E. M. F. applied tothe grid of 36. The circuit 33 excites the grids of the balancedmodulator 39, '40 differentially so that there is no output when nomodulation is applied. The screen voltage is modulated by means f thetransformer 4l, the voltage being applied differentially so as tounbalance the modulator. Referring now to Fig. 1l, which shows therelation between the screen voltage and the radio frequency voltageoutput, assume the normal voltage applied to the screens by the batteryD to be O A change in the voltage of the screens will/according to theinvention described in thq: #1,941,068 patent, add a voltage NS or NT at90 phase displacement to the output voltage MN of the tube 36. Themaximum required is about of the voltage MN to give the shift.Therefore, by properly designing the relative amplications of the tubes36 and 39, 40it is possible to produce unbalanced voltages of suchmaximum usable phase distortion due to lack of tube linearity is, en-

countered either without or with predistortion since there is nol aeroanywhere near the part of the curve which is utilized. Hence, in themodulating system of the frequency modulation transmitter described itpossible to eliminate compietely thaty type of distortion described inthe amplitude modulation case.

In the receiving system it is customary to make use of a. balanceddetector system as shown in Fig. The reactance characteristics of thetwo detector branches are shown, respectively, in Fig. 12 arranged toproduce 100% modulation over the range Fi-Fz. It will be observed thatone branch is arranged to be non-reactive at F1 and the other to benon-reactive at F2, the extremes of the frequency swing. In this way thechanges in frequency are converted completely into changes in amplitude.

It has been found that when this type of conversion system is used witha transmission which employs a predistorted output that bad distortionis encountered. This may be prevented as in amplitude modulation by areduction of the percentage of modulation at the transmitter, with theresultant loss in the signal to noise ratio. However, I nd that thetransmitter adjustment may be left unchanged, the distortion removed,and the signal to noise ratio left unimpaired provided a characteristicas illustrated in Fig. 13 is employed. By designing the characteristicsof the two branches so that the non-reactive points come at F3 and F4which are outside the range of the frequency swing F1 F2 which is beingused, complete freedom from distortion and cross modulation with noisecomponents is secured. This is accomplished Without any loss in thesignal to noise ratio provided the noise is not greater than 50% of thesignal. It is particularly valuable to be able to realize the full gainof predistortion because it is much more effective on the high frequencycomponents in frequency modulation signaling than it is on amplitudemodulation. The reason for this will appear from an examination vof thenoise characteristics of Fig. 14 which shows the relative voltagedistribution of the noise components in frequency modulation andamplitude modulation reception. It will be observed that the noisedistribution, which in the present case for purposesl of illustration isassumed to be that due solely to thermal agitation and shot effect isuniformly distributed over the audible range while that in the frequencymodulation system increases linearly from zero at zero frequency to thesame value as the amplitude modulated receiver at the upper end of theband. Hence a restorer having such a characteristic as that of Fig. 4will produce a very muchl greater reduction in the noise in thefrequency modulation system than in the amplitude modulation one. Therelative noise voltage-frequency characteristics which a. restorerhaving the characteristics of Fig. 4 willgive when applied to bothsystems is shown by the curves of Fig. 15. It will be observed that inthe range of good audibility a large improvement=in the Ireduction ofthe noise voltage is obtained. As the noise produced is proportional tothe energy involved, or the square of these values, the importance ofthe improvement is apparent. While only one form of noise h as so farbeen considered,` namely, tube and circuit noise, the general result isthe same for all sorts ofr disturbances such as ignition noise,commutatin'g machinery noise, X-ray 'machines and the like. In practiceon all these types of noise it has been found that very subt stantialvaural improvement is realized.

It will be observed that the networks shown in Fig. 3 and 4 are somewhatcomplicated. While this is of no importance at a. transmitting sta.-tion, it does become of importance when a large number of receivers areto be constructed as the element of cost is quite important. Figs. 16and l? show farms of networks which produce substantially the samecharacteristics as those shown in Figs. 3 and 4. In the predistorterwhile'a relatively large range of values may be chosen from, I find thefollowing set to be pracf tical and inexpensive.

The capacity 44 has a value of .001 'nui'."rofared the resistance l5 is'15,000 ohms. The input resistance 46 should be 300 ohms and the outputresistance 41 should be 30007ohms.

In the restorer shown in Fig. l, 48 represents the detector outputimpedance and may be of the order of 100,000 ohms. 49 is a resistance of50,- 000 ohms and 50 a capacity of .001 microfarad.

The volume control potentiometer 5I should have sists in amplifying'thehigh frequencies of the band to a substantially greater degree than thelow frequencies thereof, varying the frequency of plitude are minimized,and amplifying the lowy frequency currents of said band to asubstantially greaterdegree than the high frequency currents thereof.

2. In a system for transmitting and reproducing a band of signalingcurrents, in combination, means arranged to amplify the high frequencycurrents of the band to a substantially greater degree than the lowfrequency currents thereof, means for generating a carrier wave ofsubstantially constant frequency, means Vfor causing the amplifiedcurrents to vary over a wide range the frequency of the carrier wave,means for transmitting such wave, means for receiving the wave andamplifying the received currents, means coupled to said last-named meansfor converting the frequency variations into a. band of currents ofvariable amplitude and having 'an admittance band width substantiallygreater than the width of the said wide' range of carrier frequencies, adetectingdevicecoupled to said converting means 10 mized.

3. The combination as set forth in claim 2 ,in which the means forconverting the frequency variations into currens of variable ampliiudr`comprises two parallelpatilsyone of said paths including a. circuit,having a resonant frequency lower than the lowest frequency oi thetransmitted band of frequencies and the other of said paths including acircuit havingv a resonant, fxe- Y quency higher than the highest,

frequency of the transmitted ban.

EDWIN H. ARMSTRONG.

EPI

